In general, a non-aqueous electrolyte comprising a lithium salt such as LiPF6 and a carbonate solvent reacts continuously with the surface of a cathode active material (particularly, LiCoO2) during repeated charge/discharge cycles, resulting in the continuous formation of a resistance layer that causes an increase in resistance and interrupts conduction of Li+ ions. Such resistance layer causes the active material particles to be isolated among themselves or from a current collector (Al foil), thereby detracting from battery performance and life characteristics. Further, such problems increasingly and predominantly occur at a high temperature to accelerate side reactions between an electrolyte and the surface of a cathode when a battery is stored at a high temperature (45° C. or 60° C.) for a long time, resulting in a significant decrease in the lifetime of a battery.
Meanwhile, non-aqueous electrolyte-based secondary batteries have problems related with safety upon overcharge for the following reasons. Cathode active materials such as lithium and/or lithium ion-containing metal oxides capable of lithium ion intercalation/deintercalation are converted into thermally unstable substances due to the release of lithium during overcharge. When the temperature of a battery reaches the critical temperature, oxygen is liberated from such unstable substances and the free oxygen may react with the solvent of an electrolyte, etc., through a highly exothermic reaction mechanism. Therefore, such a series of exothermal reactions by heating results in thermal runaway.
Generally, factors affecting the safety of a battery include: (1) heat emission due to oxidation of electrolytes; and (2) heat emission resulting from the structural collapse of a cathode due to overcharge. When overcharge proceeds, heat emission occurring from the above factors independently or simultaneously causes an increase in the internal temperature of a battery, followed by ignition or explosion of the battery. Thus, batteries show a safety problem upon overcharge.
Meanwhile, when external physical impacts (for example, exposure to high temperature such as a temperature of 150° C. or higher by heating) are applied to a battery while the battery is charged or overcharged, the battery is overheated due to the heat emission caused by the reaction of an inflammable electrolyte with a cathode active material, and the structure of an electrode (particularly, a cathode) is collapsed to generate oxygen, which accelerates the combustion of the electrolyte. Therefore, a separator disposed between a cathode and an anode is melted and the electrical energy induces thermal runaway, resulting in ignition and explosion of the battery.